Hydraulic apparatus for recovering energy is defined herein as apparatus comprising:                at least one hydraulic motor;        a high-pressure fluid source; and        a low-pressure fluid source;        
the apparatus being suitable for being operated in an energy consumption mode, in which the fluid is transferred from the high-pressure fluid source to the low-pressure fluid source while driving said at least one motor in rotation; and
the apparatus being suitable for being operated in an energy accumulation mode, in which, by the action of said at least one motor operating as a pump, the fluid is transferred from the low-pressure fluid source to the high-pressure fluid source.
It can be understood that, in such apparatus, energy is stored in the high-pressure fluid source in the energy accumulation mode, and is delivered by said high-pressure fluid source in the energy consumption mode.
The term “fluid source” (high-pressure or low-pressure fluid source) is used to mean a receptacle (or a plurality of receptacles) for storing fluid, which receptacle, optionally in association with other hydraulic components, makes it possible, in an operating mode, to supply fluid under pressure at a pressure greater than atmospheric pressure. Such a fluid source can, in particular, be a hydraulic accumulator, or indeed a pressure-free fluid reservoir coupled to a pump suitable for pumping fluid from it and for delivering that fluid under pressure. It can be understood that in another operating mode (i.e. at some other time), the fluid source does not supply fluid, but rather it receives fluid in such a manner as to be filled again.
The term “hydraulic accumulator” is used to mean a storage capacity suitable for storing and for delivering a fluid under pressure, a fraction of the energy absorbed when the fluid enters the accumulator for storage therein being consumed when the fluid exits from the accumulator. A hydraulic accumulator can operate by gas compression, by means of a spring, or by some other means.
In the preceding definition, the fluid sources are referred to respectively as being “high-pressure” and “low-pressure” merely with reference to the respective parts they play in operation of the apparatus. When the apparatus is operating normally, they are subjected to pressures such that the “high-pressure” source has a pressure that is higher than the “low-pressure” source. But the two sources may be structurally identical.
When a vehicle including hydraulic energy-recovery apparatus is being designed, the volume and the weight of the accumulator require the designer of the vehicle to limit the on-board storage capacity, and to make a trade-off between the energy saving that can be made over an average operating cycle of the vehicle and the volume and/or the weight of the accumulator that is acceptable on board the vehicle.
The optimum trade-off is the one that makes it possible to minimize the weight and/or the size of the hydraulic accumulator, while also ensuring that the energy-recovery apparatus can perform the desired energy recovery and starting assistance functions.
Naturally, it is necessary, when performing such optimization, to provide continuity for the energy-recovery function (that optionally takes part in braking the vehicle), and above all for the starting assistance function.
Therefore, naturally, since the storage capacities of the accumulator are limited, as are the storage capacities of the fluid source, it can be understood that there is a risk of operation of the energy recovery system being interrupted when the level of fluid in one or the other of these storage capacities approaches allowable limit values, and in particular when the level of fluid in the high-pressure source approaches an upper limit, or when the level of fluid in the low-pressure source approaches a lower limit.
Usually, this risk is kept down to an acceptable level by providing an accumulator and a fluid source of high storage capacity, i.e. of storage capacity that is sufficiently high to ensure continuity for operation even under various operating conditions that are considered as extreme operating situations.
However, that leads to overdimensioning the accumulator, which is undesirable.